I don't know, but if they ran passenger trains, they probably ran mail CARS on the head-on. The mail contracts generally kept the passenger service going on most railroads when the passenger serice couldn't make a profit. Trains JUST for mail and express? I have no idea.
If you look at the Pocahontas Glory videos you will see trains behind J class engines in the mid 1950's comprising eight to ten heavyweight head end cars with perhaps one, or at most two, passenger cars on the end.
The "Cavalier" ran wih a lot (many!) head-end cars. Seem to remember baggage cars but can't really say about RPO's. And, somewhere on the net (I can't find it now) was a short film of two tuscan GP-9's pulling a handful of passenger cars and attached behind the passengers were a considerable string of box (express?) cars painted tuscan with what appeared to be gold lettering. Perhaps an N&W afficianado will have a definitive answer.
N&W ran head end cars on most through trains. Powathan Arrow originaly had no head end cars. Cavalier heavy on head end cars. No specific mail trains only to my knowledge. This all post war
